Qijin Chi - Academia.edu (original) (raw)

Papers by Qijin Chi

Nanoscale, Jan 21, 2014

We report a new method for controlling H- and J-stacking in supramolecular self-assembly. Graphen... more We report a new method for controlling H- and J-stacking in supramolecular self-assembly. Graphene nanosheets act as structure inducers to direct the self-assembly of a versatile organic dye, perylene into two distinct types of functional nanostructures, i.e. one-dimensional nanotubes via J-stacking and two-dimensional branched nanobuds through H-stacking. Graphene integrated supramolecular nanocomposites are highly stable and show significant enhancement of photocurrent generation in these two configurations of photosensing devices, i.e. solid-state optoelectronic constructs and liquid-junction solar cells.

The Journal of Physical Chemistry B, 2001

We have shown that Pseudomonas aeruginosa azurin can be immobilized on alkanethiol monolayers sel... more We have shown that Pseudomonas aeruginosa azurin can be immobilized on alkanethiol monolayers selfassembled on Au(111). Immobilization is achieved through hydrophobic interactions between the hydrophobic area around the copper atom in azurin and methyl heads of alkanethiol to form submonolayers or monolayers. In this orientation mode azurin molecules on Au(111) are oriented with the redox center (copper atom) facing the electrode surface. This is opposite to the orientation of azurin on bare gold which is via a surface disulfide group such as recently reported. Scanning tunneling microscopy (STM) with molecular resolution reveals that both well-ordered alkanethiol and protein adlayers are present. Adsorbed azurin molecules exhibit high stability and retain electron transfer (ET) function. Long-range interfacial ET between azurin and Au(111) across variable-length alkanethiol bridges was systematically investigated by different electrochemical techniques. Distance-dependent ET can be controlled by adjusting the length of the alkanethiol chain. The electrochemical ET rate constant is almost independent of the chain length up to ca. 9 methylene units but follows exponential distance decay with a decay factor ( ) of 1.03 ( 0.02 per CH 2 unit at longer chain lengths. Overvoltage-dependent ET was also examined. The results provide a strategy to ordered molecular assemblies, and controlled orientation and ET of azurin at atomically planar metallic surfaces. This approach can in principle be extended to other redox metalloproteins. reorganization free energy of electrochemical ET of azurin in this adsorption mode.

Journal of the Chemical Society, Faraday Transactions, 1994

Native and unfolded glucose oxidase (GOD) structures have been directly observed with scanning tu... more Native and unfolded glucose oxidase (GOD) structures have been directly observed with scanning tunnelling microscopy (STM) for the first time. STM images show an opening butterfly-shaped pattern for the native GOD. When GOD molecules are extended on anodized, highly ordered pyrolytic graphite (HOPG), a helical structure composed of double-stranded chains was obtained under STM. These results are in good agreement with previous description of the GOD molecular structure. A simple model of the unfolding process for GOD molecules was proposed to explain these observations. Electrochemical evidence was provided to support the results obtained with STM and t h e proposed model. Plate 1 STM images of GOD adsorbed on the freshly cleaved HOPG obtained with different scan scopes. (a) and (b) topographic views; (c) obtained from (b) after rotating it by 90" and reducing the scan area; (d) three-dimensional image of (c). U = 0.3 V; I = 1.2 nA; scan area = 88.33 x 88.33 nm2 (a), 31.42 x 31.42 nm2 (b), 22.82 x 22.82 nm2 (c), (d). Plate 2 Topographic views of bare HOPG electrochemically pretreated at +2.0 V for 2 min obtained in constant-height mode. U = 0.4 V; scan area = 120 x 120 nm2 (a), 10 x 10 nm2 (b). Q, Chi et al. Plate 3 scan area = lo00 x loo0 nm2 (a), 400 x 400 nm2 (b), 200.08 x 200.08 nm2 (c), 100 x 100 nm2 (d), 70.17 x 70.17 nm2 (e) and 45 x 45 nm2 (f). STM images of GOD adsorbed on the anodized HOPG at various scan scopes. ( a ) -( f ) Topographic views. U = 0.2 V; I = 1.0 nA;

Journal of the Chemical Society, Faraday Transactions, 1996

Based on scanning tunnelling microscopy and electrochemical measurements, orientation and electro... more Based on scanning tunnelling microscopy and electrochemical measurements, orientation and electrocatalytic function of riboflavin adsorbed on carbon substrates have been described for the first time. Scanning tunnelling micrographs show clearly that tip induction may result in an orientation change of the adsorbed riboflavin molecule on highly oriented pyrolytic graphite from the initially vertical orientation to the stable flat form. The adsorbed riboflavin as an effective mediator can accelerate the reduction of dioxygen which accepts two electrons from the reduced riboflavin to generate hydrogen peroxide. The rate constants of the electrocatalytic reaction in various pH solutions were determined using a rotating disc electrode modified with riboflavin. The pH effect and possible catalytic mechanism are discussed in detail.

Faraday Discussions, 2006

We address some physical features associated with long-range interfacial electron transfer (ET) o... more We address some physical features associated with long-range interfacial electron transfer (ET) of metalloproteins in both electrochemical and electrochemical scanning tunneling microscopy (ECSTM) configurations, which offer a brief foundation for understanding of the ET mechanisms. These features are illustrated experimentally by new developments of two systems with the blue copper protein azurin and enzyme nitrite reductase as model metalloproteins. Azurin and nitrite reductase were assembled on Au(111) surfaces by molecular wiring to establish effective electronic coupling between the redox centers in the proteins and the electrode surface for ET and biological electrocatalysis. With such assemblies, interfacial ET proceeds through chemically defined and well oriented sites and parallels biological ET. In the case of azurin, the ET properties can be characterized comprehensively and even down to the single-molecule level with direct observation of redox-gated electron tunnelling resonance. Molecular wiring using a p-conjugated thiol is suitable for assembling monolayers of the enzyme with catalytic activity well-retained. The catalytic mechanism involves multiple-ET steps including both intramolecular and interfacial processes. Interestingly, ET appears to exhibit a substrate-gated pattern observed preliminarily in both voltammetry and ECSTM.

Scientific Reports, 2015

Quantum dots (QDs) and graphene are both promising materials for the development of new-generatio... more Quantum dots (QDs) and graphene are both promising materials for the development of new-generation optoelectronic devices. Towards this end, synergic assembly of these two building blocks is a key step but remains a challenge. Here, we show a one-step strategy for organizing QDs in a graphene matrix via interfacial self-assembly, leading to the formation of sandwiched hybrid QD-graphene nanofilms. We have explored structural features, electron transfer kinetics and photocurrent generation capacity of such hybrid nanofilms using a wide variety of advanced techniques. Graphene nanosheets interlink QDs and significantly improve electronic coupling, resulting in fast electron transfer from photoexcited QDs to graphene with a rate constant of 1.3 × 10 9 s −1 . Efficient electron transfer dramatically enhances photocurrent generation in a liquid-junction QD-sensitized solar cell where the hybrid nanofilm acts as a photoanode. We thereby demonstrate a cost-effective method to construct large-area QD-graphene hybrid nanofilms with straightforward scale-up potential for optoelectronic applications.

Nanostructure Science and Technology, 2009

... Qijin Chi, Palle S. Jensen, and Jens Ulstrup ... Electron transfer patterns of cyt c confined... more ... Qijin Chi, Palle S. Jensen, and Jens Ulstrup ... Electron transfer patterns of cyt c confined to various functionalized SAMs on gold and silver electrodes have been systematically studied by electrochemistry and different types of spectroscopies (Tarlov and Bowden 1991; Song ...

Supplementary Figure S1. Chemical structure and molecular dimensions of the thiolated ligand used... more Supplementary Figure S1. Chemical structure and molecular dimensions of the thiolated ligand used in this work, 4´-(6-acetylthiohexyloxy)-2, 2´: 6´ 2´´-terpyridine. Supplementary Figure S2. Voltammetric measurements: (a) cyclic voltammograms of Os-terpy SAMs on Au(111) surfaces obtained at different scan rates of 0.5, 0.75, 1.0, 2.0, 5.0 V s -1 and (b)

Langmuir, 2014

The triazatriangulene (TATA) ring system was investigated as a binding group for tunnel junctions... more The triazatriangulene (TATA) ring system was investigated as a binding group for tunnel junctions of molecular wires on gold surfaces. Self-assembled monolayers (SAMs) of TATA platforms with three different lengths of phenylene wires were fabricated, and their electrical conductance was recorded by both conducting probe-atomic force microscopy (CP-AFM) and scanning tunneling microscopy (STM). Similar measurements were performed for phenylene SAMs with thiol anchoring groups as references. It was found that, despite the presence of a sp 3 hybridized carbon atom in the conduction path, the TATA platform displays a contact resistance only slightly larger than the thiols. This surprising finding has not been reported before and was analyzed by theoretical computations of the transmission functions of the TATA anchored molecular wires. The relatively low contact resistance of the TATA platform along with its high stability and directionality make this binding group very attractive for molecular electronic measurements and devices.

Journal of Physical Chemistry C, 2013

Many key chemical and biochemical reactions, particularly in living cells, take place in confined... more Many key chemical and biochemical reactions, particularly in living cells, take place in confined space at the mesoscopic scale. Toward understanding of physicochemical nature of biomacromolecules confined in nanoscale space, in this work we have elucidated fluorescence effects of a light harvesting complex LH2 in nanoscale chemical environments. Mesoporous silicas (SBA-15 family) with different shapes and pore sizes were synthesized and used to create nanoscale biomimetic environments for molecular confinement of LH2. A combination of UV−vis absorption, wide-field fluorescence microscopy, and in situ ellipsometry supports that the LH2 complexes are located inside the silica nanopores. Systematic fluorescence effects were observed and depend on degree of space confinement. In particular, the temperature dependence of the steady-state fluorescence spectra was analyzed in detail using condensed matter band shape theories. Systematic electronicvibrational coupling differences in the LH2 transitions between the free and confined states are found, most likely responsible for the fluorescence effects experimentally observed.

ChemInform, 2008

Single-Molecule Electron Transfer in Electrochemical Environments -[ca. 450 refs.]. -(ZHANG, J.; ... more Single-Molecule Electron Transfer in Electrochemical Environments -[ca. 450 refs.]. -(ZHANG, J.; KUZNETSOV, A. M.; MEDVEDEV, I. G.; CHI, Q.; ALBRECHT, T.; JENSEN, P. S.; ULSTRUP*, J.; Chem. Rev. (Washington, D. C.) 108 (2008) 7, 2737-2791; Dep. Chem., Tech. Univ. Den., DK-2800 Lyngby, Den.; Eng.) -Lindner 42-278

Ultramicroscopy, 2009

J. Ulstrup). 1 These authors have equal contributions. Ultramicroscopy ] (]

The Journal of Physical Chemistry C, 2009

Self-assembled monolayers (SAMs) formed by chemisorption of a branched-chain alkanethiol, 2-methy... more Self-assembled monolayers (SAMs) formed by chemisorption of a branched-chain alkanethiol, 2-methyl-1propanethiol, on Au(111) surfaces were studied by in situ scanning tunneling microscopy (STM) under electrochemical potential control and analyzed using extensive density functional theory (DFT) calculations. The SAM forms in the unusual (8 × 3)-4 superlattice, producing a very complex STM image. Seventy possible structures were considered for the SAM, with the calculated lowest-energy configuration in fact predicting the details of the unusual observed STM image. The most stable structure involves two R-S-Au-S-R adatom-mediated motifs per surface cell, with steric-induced variations in the adsorbate alignment inducing the observed STM image contrasts. Observed pits covering 5.6 ( 0.5% of the SAM surface are consistent with this structure. These results provide the missing link from the structural motifs observed on surfaces at low coverage and on gold nanoparticles to the observed spectroscopic properties of high-coverage SAMs formed by methanethiol. However, the significant role attributed to intermolecular steric packing effects suggests a lack of generality for the adatom-mediated motif at high coverage.

The Journal of Physical Chemistry B, 2010

Intramolecular electron transfer (ET) between metal centers is a core feature of large protein co... more Intramolecular electron transfer (ET) between metal centers is a core feature of large protein complexes in photosynthesis, respiration, and redox enzyme catalysis. The number of microscopic redox potentials and ET rate constants is, however, prohibitive for experimental cooperative ET mapping, but two-center proteins are simple enough to offer complete communication networks. At the same time, multicenter redox proteins operate in membrane environments where conformational dynamics may lead to gated ET features different from conditions in homogeneous solution. The bacterial respiratory diheme protein Pseudomonas stutzeri cytochrome c 4 has been a target for intramolecular, interheme ET. We report here voltammetric and in situ scanning tunneling microscopy (STM) data for P. stutzeri cyt c 4 at single-crystal, atomically planar Au(111)electrode surfaces modified by variable-length ω-mercapto-alkanoic carboxylic acids. As evidenced by in situ STM, the strongly dipolar protein is immobilized in a close to vertical orientation at this surface with the positively charged high-potential heme domain adjacent to the electrode. This orientation gives asymmetric voltammograms with two one-ET peaks in the cathodic direction and a single two-ET peak in the anodic direction. Intramolecular, interheme ET with high, 8,000-30,000 s -1 , rate constants is notably an essential part of this mechanism. The high rate constants are in striking contrast to ET reactions of P. stutzeri cyt c 4 with small reaction partners in homogeneous solution for which kinetic analysis clearly testifies to electrostatic cooperative effects but no intramolecular, interheme ET higher than 0.1-10 s -1 . This difference suggests a strong gating feature of the process. On the basis of the three-dimensional structure of P. stutzeri cyt c 4 , gating is understandable due to the through-space, hydrogen-bonded electronic contact between the heme propionates which is highly sensitive to environmental configurational fluctuations.

Surface Science, 2000

A novel method has been developed to fabricate nanoscale pits on Au(111) in aqueous environments ... more A novel method has been developed to fabricate nanoscale pits on Au(111) in aqueous environments by in situ scanning tunnelling microscopy (STM ), based on critical interactions between tip and substrate. The most striking advantages of the present method are that the dimension and position of the pits can be controlled well in aqueous environments, and the operations are simple. Parameters affecting the pit formation and size have been systematically characterized to show that pit formation is dominated by bias voltage. A mechanism is proposed based on local surface reconstruction induced by electronic contact between tip and substrate.

Proceedings of the National Academy of Sciences, 2005

A biomimetic long-range electron transfer (ET) system consisting of the blue copper protein azuri... more A biomimetic long-range electron transfer (ET) system consisting of the blue copper protein azurin, a tunneling barrier bridge, and a gold single-crystal electrode was designed on the basis of molecular wiring self-assembly principles. This system is sufficiently stable and sensitive in a quasi-biological environment, suitable for detailed observations of long-range protein interfacial ET at the nanoscale and single-molecule levels. Because azurin is located at clearly identifiable fixed sites in well controlled orientation, the ET configuration parallels biological ET. The ET is nonadiabatic, and the rate constants display tunneling features with distance-decay factors of 0.83 and 0.91 Å ؊1 in H2O and D2O, respectively. Redoxgated tunneling resonance is observed in situ at the single-molecule level by using electrochemical scanning tunneling microscopy, exhibiting an asymmetric dependence on the redox potential. Maximum resonance appears around the equilibrium redox potential of azurin with an on͞off current ratio of Ϸ9. Simulation analyses, based on a two-step interfacial ET model for the scanning tunneling microscopy redox process, were performed and provide quantitative information for rational understanding of the ET mechanism.

Proceedings of the National Academy of Sciences, 2001

Intramolecular electron transfer in azurin in water and deuterium oxide has been studied over a b... more Intramolecular electron transfer in azurin in water and deuterium oxide has been studied over a broad temperature range. The kinetic deuterium isotope effect, k H͞kD, is smaller than unity (0.7 at 298 K), primarily caused by the different activation entropies in water (؊56.5 J K ؊1 mol ؊1 ) and in deuterium oxide (؊35.7 J K ؊1 mol ؊1 ). This difference suggests a role for distinct protein solvation in the two media, which is supported by the results of voltammetric measurements: the reduction potential (E 0 ) of Cu 2؉͞؉ at 298 K is 10 mV more positive in D2O than in H2O. The temperature dependence of E 0 is also different, yielding entropy changes of ؊57 J K ؊1 mol ؊1 in water and ؊84 J K ؊1 mol ؊1 in deuterium oxide. The driving force difference of 10 mV is in keeping with the kinetic isotope effect, but the contribution to ⌬S ‡ from the temperature dependence of E 0 is positive rather than negative. Isotope effects are, however, also inherent in the nuclear reorganization Gibbs free energy and in the tunneling factor for the electron transfer process. A slightly larger thermal protein expansion in H 2O than in D2O (0.001 nm K ؊1 ) is sufficient both to account for the activation entropy difference and to compensate for the different temperature dependencies of E 0 . Thus, differences in driving force and thermal expansion appear as the most straightforward rationale for the observed isotope effect.

Nature Communications, 2013

Coordination chemistry has been a consistently active branch of chemistry since Werner's seminal ... more Coordination chemistry has been a consistently active branch of chemistry since Werner's seminal theory of coordination compounds inaugurated in 1893, with the central focus on transition metal complexes. However, control and measurement of metal-ligand interactions at the single-molecule level remain a daunting challenge. Here we demonstrate an interdisciplinary and systematic approach that enables measurement and modulation of the coordinative bonding forces in a transition metal complex. Terpyridine is derived with a thiol linker, facilitating covalent attachment of this ligand on both gold substrate surfaces and gold-coated atomic force microscopy tips. The coordination and bond breaking between terpyridine and osmium are followed in situ by electrochemically controlled atomic force microscopy at the single-molecule level. The redox state of the central metal atom is found to have a significant impact on the metal-ligand interactions. The present approach represents a major advancement in unravelling the nature of metal-ligand interactions and could have broad implications in coordination chemistry.